The present invention relates to thermoelectric coolers, and more particularly to a control circuit for regulating the temperature of a laser or other device coupled to a thermoelectric cooler.
Temperature control of electronic devices is an important design consideration. Solid-state components such as transistors generate heat in operation. Excessive amounts of heat can damage the component or other sensitive elements of a circuit in close proximity.
With advances in optical technologies, such as optical fiber communication, electrical signal processing is being replaced in many applications by optical signal processing. For example, cable television signals previously transmitted over wire (i.e., coaxial cable) are now capable of being carried on optical fiber networks. Optical transmission technologies provide greater bandwidth, better signal-to-noise ratio, and potentially lower cost.
In order to transmit a signal over an optical fiber, lasers are used to provide a coherent light source. The lasers generate a substantial amount of heat, and must be cooled to prevent failure. Pump lasers in particular require a great deal of cooling for reliable operation.
In cable television applications, lasers are provided along the communication network for signal amplification. Signal distribution amplifiers are mounted on utility poles and must withstand severe environmental conditions. The internal temperature of the amplifier housing can range, for example, from -20.degree. C. to 90.degree. C. The distribution amplifiers must be designed to operate over this entire temperature range. In order to maintain the temperature of the lasers used in the amplifiers within their design specifications, cooling and heating devices are required.
One device that can be used to regulate the temperature of a laser is a thermoelectric cooler ("TEC"). These devices are small heat pumps that obey the laws of thermodynamics as do conventional mechanical heat pumps such as refrigerators. TECs, however, are solid-state devices that operate as Peltier coolers. A single-stage thermoelectric cooler is composed of a matrix of thermoelectric couples, connected electrically in series and thermally in parallel. When current is conducted through the thermoelectric couples in one direction, a first plate of the TEC cools down and a complementary second parallel plate heats up. When current is conducted in the other direction, the first plate heats up and the second plate cools down.
TEC cooling is proportional to the applied current. The power dissipated by Joule heating in the TEC is proportional to the square of the current. Thus, an increase in current above a certain value will result in less net cooling because the Joule heating is increasing at a faster rate than the Peltier cooling. It is therefore important to carefully control the current that is applied to the TEC.
Known control circuits for regulating the temperature of a thermoelectric cooler use a linear driver to input current to the cooler. Because of their design, the operation of linear drivers can be very inefficient, generating additional heat that must be dissipated into a heat sink. Since they operate in a linear fashion, these drivers are always drawing current and must dissipate power constantly. Such operation defeats the cooling function of the TEC, and is particularly damaging in a cable television fiber distribution amplifier that encloses the circuitry in a heat retaining weather-tight box.
It would be advantageous to provide a temperature control circuit for a thermoelectric cooler that operates in a nonlinear fashion. Both heating and cooling of a device to be protected should be provided in a reliable and efficient manner. It would be further advantageous to provide such a circuit that operates using the same voltages already present in the circuitry for the device being protected. The present invention provides a control circuit and method for driving a thermoelectric cooler having these advantages.